Power amplifiers are key components of mobile and satellite communications systems. To increase efficiency of such systems, signals of multiple channels are amplified by a common power amplifier. Power amplifiers that operate outside of a small signal range are inherently nonlinear and thus introduce signal distortion when multiple signals are amplified. Moreover, to achieve high efficiency, power amplifiers in communications applications must operate near saturation levels. However, as amplitude and phase distortion drastically increase near a saturation region, highly efficient power amplifiers must be linearized for satisfactory operations.
Predistortion and feed-forward approaches have been used for linearizing power amplifiers. Feed-forward amplifier circuits typically employ a main amplifier which produces a fundamental power component and an unwanted distortion power component. A correction amplifier is used to produce only the distortion power component. An output combining circuit combines signals produced by the both amplifiers to cancel the distortion component. While a feed-forward linearizer performs satisfactorily for some transmitter systems, it is very expensive and requires critical alignment.
Predistortion is another technique for error correction in power amplifiers. In effect, the input signal to a power amplifier is predistorted with distortion components having phase and amplitude preselected so as to cancel distortion components introduced by the power amplifier. In a typical predistorion linearizer, the input signal is split into two paths: a direct path and a predistortion path. In the predistortion path, the input signal is conditioned to produce a predistortion signal which is combined with the signal produced in the direct path. The combined signal is applied to a main amplifier. If the amplitude and phase of the predistortion signal are properly selected, the output of the main amplifier will have a low level of distortion.
FIG. 1 illustrates a conventional microwave power amplifier used in a portable telephone system. Input circuitry of the power amplifier comprises an input high power isolator 2, a predistortion linearizer 4 provided to compensate for the distortion of the power amplifier, and a high power isolator 6 arranged at the output of the linearizer 4. Ferrite circulators are usually employed as high power isolators. An attenuator 8 is coupled to the output isolator 6 for adjusting an input power level. The attenuator 8 can be controlled to provide temperature compensation of the input signal. A main power amplifier 10 is connected to the output of the attenuator 8.
Referring to FIG. 2, the predistortion linearizer 4 comprises a series feedback amplifier that employs a common-source field-effect transistor (FET) with a large source inductance Ls. An input matching circuit 12 is arranged at the input of the series feedback amplifier, and an output matching circuit 14 is provided at its output.
Another known implementation of the predistortion linearizer 4 is illustrated in FIG. 3, which shows a series diode linearizer having a diode D connected in parallel with a capacitor Cp. Bias chokes Lb1 and Lb2 are arranged in a direct-current power supply circuit. Capacitors Cb1 and Cb2 are arranged at the input and output of the linearizer to provide direct-current (DC) blocking. The series diode linearizer causes positive amplitude and negative phase deviations when the input power increases to compensate for distortions introduced by a power amplifier.
A further example of a conventional predistortion linearizer 4 is presented in FIG. 4, which illustrates a parallel diode linearizer using nonlinear resistance characteristics of a diode in forward bias condition for providing compensation for distortions. The parallel diode linearizer comprises a diode D coupled in parallel with a circuit between the input and the output of the linearizer. The diode D can be represented by an equivalent variable resistance Rd and a junction capacitance Cj connected in parallel. A resistor Rb is coupled between the diode D and a voltage supply Vcc to provide direct-current bias. Capacitors Cb1 and Cb2 provide direct-current blocking at the input and output of the linearizer.
As discussed above, in a conventional power amplifier illustrated in FIGS. 1-4, a predistortion linearizer requires high power isolators at its input and output. Therefore, it cannot be integrated with the main power amplifier to reduce the size of the amplification system.
FIG. 5 illustrates another example of a conventional linearizer for a power transistor amplifier. The linearizer comprises a forward biased base-collector diode HBT2, which together with a resistor Rb forms a base biasing circuit for a power transistor HBT1 used to amplify a radio frequency (RF) signal. A separate bias terminal Vbb is provided in addition to input and output terminals of the power amplifier to provide DC bias.
As the input power supplied to the base of the transistor HBT1 increases, the rectified DC current of the base-collector diode HBT1 increases and the DC voltage across the diode decreases. As a result, the DC voltage across the base-emitter diode HBT2 increases. The increased DC current drives the transistor HBT1 stronger, reducing the gain compression and phase distortion of the power amplifier. Since the DC voltage across the base-collector diode HBT2 decreases with the increase of the input power, the differential conductance of the diode decreases.
As the base-emitter conductance of the transistor HBT1 increases with the increase of the input power, the base-collector diode HBT2 compensates for variations of the HBT1 base-emitter conductance under large signal conditions, and reduces the phase distortion.
As discussed above, the linearizer shown in FIG. 5 requires a separate bias voltage terminal Vbb in addition to regular terminals used for a power transistor. Therefore, this linearizer cannot be integrated into a standard power transistor package.
However, to provide compatibility with other elements of a communications system, it would be desirable to create a linearizer that would be "invisible" to system elements coupled to a power transistor, and therefore, could be integrated into a standard power transistor package.
Also, it would be desirable to create a power transistor package containing a linearizer and having no additional terminals compared to standard transistor packages.